My UHaul CT13 is in winter storage now, so I can't give you an exact measurement. But I am 6'1-1/2" and can stand straight inside without touching the ceiling. I did have to duck a bit when the original swamp cooler was still there, but replacing it with a FanTastic fan eliminated that problem. So I'd guess 6'2" would be about it for headroom.

Ditto here. Six ft. one in. hubby has to duck for the air conditioner but otherwise, except for the curved sides can stand up. I am 5'7" and only bump my head when I forget to duck upon exiting the camper.